METHOD AND APPARATUS FOR ACCOMPLISHING GAS-FLOW INTO THE GAP BETWEEN STATOR AND ROTOR OF AN ELECTRIC MACHINE

Abstract: The invention relates to a method for accomplishing a gas flow into the gap (8) between the stator and the rotor of an electric machine. The gas, advantageoulsy at least partly air, is pre-accelerated before entering the gap (8) and the gas is conveyed to the gap (8) substantially at a flow rate corresponding to the pre-acceleration. The pre-acceleration is carried out advantageously to a rate that is substantially half the peripheral speed of the rotor (1) and substantially in the tangential direction of the periphery of the rotor (1). The invention relates also to an apparatus for applying the method.

Description: Method and apparatus for accomplishing gas flow into the gap between stator and rotor of an electric machine The invention relates to a method for accomplishing a gas flow into the gap between stator and rotor of an electric machine according to the preamble of claim 1. With increasing peripheral speed of the rotor of an electric machine, also gas frictional losses are highly increased in the gap for free gas flow be¬ tween the stator and rotor. For decreasing gas frictional losses in large electric machines, e.g. in a 300 MW generator, a gas filling is used in the gap for free gas flow, wherein the gas used is as light in weight as pos¬ sible, usually hydrogen. Thus the gas frictional losses can be decreased to a fractional part in comparison with the situation where the gap is filled by a heavier gas, such as air. In this manner the gas frictional losses are controlled to a reasonable extent particularly in large electric machines with a high peripheral speed of the rotor. In machines with relatively small output and high peripheral rotor speed, i.e. in so-called high-speed machines, however, the use of a light-weight gas, such as hydrogen or helium, is not reasonable in view of the operation and operating economy of the electric machine as a whole, because an inevitable small leak of a light-weight gas will induce unreasonably high operating costs in relation to the output of the electric machine. As to prior art, reference is made to the publication DE-A1 -3314426 disclosing a construction, in which the rotor is equipped with a set of blades leading a cooling agent on the periphery of the rotor in a direction opposite to the direction of rotation of the rotor. This kind of a structural solution is complicated, particularly in relatively small electric machines, because it requires special structures in the rotor, which is a demanding object of planning as such, as a revolving machine part and in view of applying the basic functions. The purpose of this invention is to present a method by which a gas flow for cooling the electric machine can be achieved by structurally simple measures in the gap for free gas flow between the stator and the rotor of an electric machine in a manner that the gas frictional losses induced are reasonable. In particular, the invention can be advantageously applied in electric machines with relatively small output and the rotor revolving at a high peripheral speed, i.e. so-called high-speed electric machines. The method of the invention is primarily characterized in that the pre- acceleration is carried out in stages in a manner that the gas to be supplied to the gap for free gas flow is pressurized outside the rotor, and the gas is transferred to means for increasing the gas flow rate in the stator and in connection with the gap for free gas flow, such as expansion nozzles, where the gas is pre-accelerated to be supplied to the gap for free gas flow. By pre-acceleration, the additional loss is avoided which would be in- duced if the gas were accelerated to the rate prevalent in the gap first after entering the gap. It is true that a minor loss is induced by the fact that the pre-accelerated gas is not in the speed profile prevalent in the gap for free gas flow, wherein the speed profile of the pre-accelerated gas is modified to correspond to the speed profile of the gas flowing freely in the gap. As the cooling gas supplied to the gap for free gas flow (air gap), heavier gases can be used, such as air or the like. According to the in¬ vention, the rotor construction demanding careful planning can be con¬ structed without channels and guiding blades required for pre-accelera¬ tion. It is possible to construct the stator, a non-revolving machine part, in a structurally simple manner to incorporate the necessary channel sys¬ tems and nozzle constructions. The average flow rate of the gas in the gap between the stator and the rotor is half of the peripheral speed of the rotor, wherein it is advan- tageous that pre-acceleration is carried out to the speed that is substan¬ tially half of the peripheral speed of the rotor. Thus the frictional loss in¬ duced by the change of the gas speed profile can be minimized. In the context of this invention, particularly the terms "high-speed electric machine" and "high peripheral speed" refer to such physical conditions under which the maximum rate of the gas flowing in the gap becomes substantially close to the rate of sound in the respective gas. In ideo¬ logical terms, the flow rate of the gas on the rotor surface is substantially the same as the peripheral speed of the rotor on the outer surface of the rotor. The advantages of the invention are essentially revealed under conditions, where the ratio A, i.e. the speed of sound in gas / peripheral speed of the rotor is greater than ca. 0.8. The speed of sound in the air being ca. 330 m/s in room temperature and ca. 305 m/s in -20°C, the peripheral speed value on the outer surface of the rotor, from which on the invention brings substantial advantage, is ca. 240 m/s. In a known manner, the speed of sound in the air increases in proportion to the square of the absolute temperature. In a cold medium (e.g. in freons), the speed of sound is ca. 160 m/s and the corresponding peripheral speed value is ca. 120 m s. The invention can thus be applied also in larger electric machines, wherein it is the ratio A that is decisive. The usability of the invention can also be affected by the choice of the gas composition, as indicated by the ratio A. The pre-accelerated gas is advantageously supplied to the gap for free gas flow substantially in a direction tangential to the outer periphery of the rotor. By this solution, the losses caused by the change in the speed profile are further decreased, because the main flow direction of the gas in the gap is substantially tangential. It is advantageous to carry out the pre-acceleration in stages so that at the first stage, the gas to be supplied to the gap is pressurized and at the second stage, the gas is transferred to expansion means in connection with the gap for free gas flow, such as to expansion nozzles, where the gas is pre-accelerated to be supplied in the gap. It is advantageous that the gas is intercooled between pressurization and expansion for lowering the temperature of the gas, wherein the heat generated upon pressuriza¬ tion can be discharged by said intercooiing, and simultaneously the effect of intercooiing the pre-accelerated gas to be supplied in the gap can be increased. The invention relates further to an apparatus for accomplishing gas flow into the gap between stator and rotor of an electric machine as defined in the preamble of the independent claim on the apparatus. The apparatus is primarily characterized in that the means for pre-accelerating the gas comprise: at least one means, separate from the rotor, for increasing the pres¬ sure of the gas, such as a blower wheel or the like, and means in connection with the stator for increasing the speed gas flow rate, such as expansion nozzles, by which the gas is pre-accelerated into the rate of supply into the gap for free gas flow. Using this method gives the above-mentioned technical advantages. The invention will be illustrated in more detail in the following description with reference to the appended drawings. In the drawings, Fig. 1 shows the structure of the apparatus according to the invention in a schematic cross-sectional view in the axial direction, Fig. 2 shows the section 11- 11 of Fig. 1 , and Fig. 3 shows the reference apparatus described in Example 1, also in schematic cross-sectional view in the axial direction. With reference to Fig. 1, the apparatus of the invention is placed in con¬ nection with an electric machine, particularly an electric machine revolving at a high peripheral speed. The electric machine has a rotor 1 placed on a rotor shaft 2. At the location of the rotor 1 , a stator 4 is anchored in the frame 3 of the electric machine to surround the rotor 1. The apparatus according to the invention comprises a pressure-increasing means 5 which is preferably placed on the rotor shaft 2 to be driven by the rotor shaft 2. In terms of structure, the pressure-increasing means 5 can naturally be a blower wheel or a corresponding pressurizing means operating on the kinetic principle. The gas, particularly air, enters the pressure-increasing means 5 under a pressure corresponding to that of atmospheric air (1.0 bar abs). The pressurized gas is conveyed through an intercooler 6 to means 7 for increasing the gas flow rate. The means for increasing the gas flow rate are in an advantageous manner primarily expansion nozzles 16 or the like, by which the pressurized gas, cooled by the intercooler 6 to a temperature of e.g. 20°C, is pre-accelerated to the rate of supply into the gap 8 for free air flow. The means 7 for increasing the gas flow rate comprise also a passage 11 or the like in connection with the expansion nozzles 16, through which the gas is supplied to the expansion nozzles 16. The means for increasing the gas flow rate are preferably placed at the ends of the stator outside the stator. The pre- cooied gas is conveyed through the channel system 9 (shown schemati- cally) to both means 7 for increasing the gas flow rate. The gas is discharged from the gap 8 for free gas flow by using means 10 which comprise preferably a channel system in the radial direction, penetrating the stator 4 at its center part, wherein the gas to be fed from both ends of the stator to the gap 8 flows a distance of substantially equal length to the means 10. Each of the two means 7 for increasing the gas flow rate comprises a passage 11 in the radial direction which is limited in the first axial direction by the radial end surface 12 of the stator 4. In the second axial direction, a radial end plate 13 is arranged at a distance from said end surface 12 and has its face 14 at the point of the rotor 1 so that a slit 15 is left between said face 14 and the outer surface of the rotor 1 , wherein a minor gas flow takes place that cooles the end structures of the electric machine (not shown). At the inner end of the passage 11 , which is thus substantially ring-like in form, expansion nozzles 16 are provided for the gas flow rate, formed as a ring-like structure surrounding the rotor 1. Fig. 2 shows particularly the structure of expansion nozzles 16 in the sec¬ tion ll-ll of Fig. 1 , i.e. seen from the axial direction. The trailing edges of the blade system of the expansion nozzles 16 are in the radial direction substantially at the point of the inner diameter of the stator 4, and the ex¬ pansion nozzles are so directed that the pre-accelerated gas passes to the gap 8 parallel to the arrow 17 substantially in the tangential direction of the rotor 1 , wherein the gas flow rate is, in terms of the indices of Fig. 2, 1 / x U, when U indicates the peripheral speed of the rotor. The blade sys¬ tem 18 forming the expansion nozzles 16, as shown in Fig. 2, is arranged to surround the entire outer periphery of the rotor 1 so that the blades of the blade system 18 are arranged at certain intervals in the direction of the periphery to allow the gas to flow between them. The blade system is supported by the walls of the passage 11 , i.e. the end surface 12 of the stator 4 and the inner surface of the end plate 13. As mentioned before, the ratio A, i.e. the speed of sound in the pre-accelerated gas/the peripheral speed of the outer surface of the rotor, is greater than 0.8. The advantages of the method and apparatus according to the invention will be illustrated further in the following examples. EXAMPLE 1. In this calculation example, an electric machine according to Fig. 3 is used, provided with a rotor 1 having a length of 255 mm and a radius of 35 mm. The length of the stator 4 in the axial direction of the rotor 1 is 175 mm. Via the channel system 19 in the stator 4, air is supplied through the frame pipe 3 at the temperature of 20°C and under atmospheric pres¬ sure, wherein the air mass flow rate is 0.092 kg/s. From the central chan¬ nel system of the stator 4, the gas enters the gap 8, and at the ends of the stator the air temperature is 165°C and the temperature of the discharged gas is 110°C. The reference numerals are essentially identical with those in Fig. 1. The revolving speed of the rotor 1 is 150 000 rpm, wherein the peripheral speed is 550 m/s. When the gas entering the gap 8 is under atmospheric pressure, the gas frictional loss induced at the rotor at its ends is 6652 W, of which 3983 W is induced in the area of the gap. In addition to this, electrical losses of 1250 W are induced, so that the power output from the gap is 5233 W. The power transmission is carried out in the structure of Fig. 1 by using cold air under atmospheric pressure. However, this air must be accele¬ rated in the gap for free gas flow to a peripheral flow rate corresponding in average to half of the peripheral speed of the rotor. Acceleration to this tangential flow rate involves additional losses. These losses can be calcu¬ lated to total 13 200 W. From this lost output, 2/3 is transformed to heat in the gap for free gas flow, so that the power output from the gap increases to a value of 14 000 W. Consequently, the situation is in a way running in a circle in this kind of structures; increasing the cooling flow entails an in- crease in the gas frictional losses, and achieving the desired cooling effect often requires an unreasonable high mass flow of the gas and the power transfer ratio becomes low. With the said air mass flow, the air temperature at the end of the gap, i.e. at the ends of the stator 4, is 165°C. This is too high a value, particularly in view of the fact that after the gap, the temperature of the air is still raised to a value of 210°C, when the velocity energy of the gas is transformed to heat. With the values pre¬ sented above, the power transfer ratio of the electric machine becomes 78%, but since the air flow must still be increased, the effective output is perhaps 70%. EXAMPLE 2. The calculation example of Fig. 1 corresponds in its primary dimensions with that shown in Fig. 3. The air mass flow elected for the pre-accele¬ rated gas to be blown into the gap 8 is still 0.092 kg/s, its pressure upon entering the passage 11 being 1.62 bar abs. Thanks to the pre-accelera¬ tion, the static temperature of the air upon entering the gap is -16°C. The heat output from the gap, considering the change of the speed profile, is now 6393 W, wherein the temperature of the air in the middle of the stator 4 just before exiting to the channel system 10 is 50°C. In the central passage, the air temperature raises to a value of 96°C, the velocity energy being transformed to heat. The total power transfer ratio, including the power required by the means 5 for increasing the gas pressure, will be 85%. The temperature of the air in the gap is in this case so low that the mass flow can even be reduced and thus the power transfer ratio can be further improved. It is advantageous to manufacture the expansion nozzle structure e.g. by engraving.

Claims: Claims: 1. Method for accomplishing a gas flow into the gap (8) between the stator and the rotor of an electric machine, wherein the gas, advan- tageously at least partly air, is pre-accelerated before entering the gap (8) and the gas is conveyed to the gap (8) substantially at a flow rate cor¬ responding to the pre-acceleration, characterized in that the pre- acceleration is carried out in stages in a manner that - the gas to be supplied to the gap (8) for free gas flow is pressurized outside the rotor, and the gas is transferred to means (7) in connection with the stator for increasing the gas flow rate and connected with the gap (8), such as expansion nozzles, where the gas is pre-accelerated to be supplied into the gap. 2. Method according to claim 1 , characterized in that the pre- acceleration is carried out to a rate that is substantially half of the peri¬ pheral speed of the rotor (1). 3. Method according to claim 1 or 2, characterized in that the range of revolving speed of the rotor (1) is so elected that the ratio A > 0.8, wherein the ratio A is the speed of sound in the pre-accelerated gas / the peripheral speed of the rotor. 4. Method according to any of claims 1-3, characterized in that the pre-accelerated gas is conveyed into the gap (8) substantially in the tangential direction of the outer periphery of the rotor (1). 5. Method according to any of claims 1-4, characterized in that the gas is intercooled between pressurization and expansion to decrease the temperature of the gas. 6. Method according to any of claims 1-5, characterized in that the gas is conveyed in pre-accelerated state to the gap (8) at least from one end of the stator (4). 7. Method according to any of claims 1-6, characterized in that the gas is conveyd to the gap (8) in pre-accelerated state from both ends of the stator (2) and the gas is discharged from the middle of the stator. 8. Apparatus for accomplishing a gas flow into the gap (8) be¬ tween the stator and the rotor of an electric machine, wherein the appara¬ tus comprises means (5, 7) for pre-accelerating the gas to be supplied to the gap (8) and for conveying the gas to the gap (8) substantially at a flow rate corresponding to the pre-acceleration, characterized in that the means for pre-accelerating the gas comprise: at least one means (5), separate from the rotor, for increasing the pressure of the gas, such as a blower wheel or the like, and means (7) in connection with the stator for increasing the gas flow rate, such as expansion nozzles, by which the gas is pre-accelerated into the rate of supply into the gap (8) for free gas flow. 9. Apparatus according to claim 8, characterized in that the means for pre-accelerating the gas comprise further intercooiing means (6) for cooling the gas, wherein the intercooiing means are placed in the direction of the gas flow after said at least one pressurizing means (5) before the means (7) for increasing the gas flow rate. 10. Apparatus according to claim 8 or 9, characterized in that the means (7) for increasing the gas flow rate are placed in connection with at least one stator end. 11. Apparatus according to any of the claims 8 to 10, character¬ ized in that the means (7) for increasing the gas flow rate are placed at both ends of the stator and the means (10) for discharging the gas from the gap (8) for free gas flow are placed in the middle part of the stator (4). 12. Apparatus according to any of the claims 8 to 11, character¬ ized in that the means (7) for increasing the gas flow rate comprise a radial passage (11) or the like which is limited in the first axial direction by a radial end surface (12), or the like, of the stator (4) and in the second axial direction by a substantially radial end plate (13) or the like arranged at a distance from said end surface (12) or the like, formed as a ring-like structure in a manner that the trailing edges of the expansion nozzles (16) or the like are in the radial direction substantially at the point of the inner diameter of the stator (4). 13. Apparatus according to any of the claims 8 to 12, character¬ ized in that the expansion nozzles (16) or the like are so directed that they give the pre-accelerated gas substantially the rate of the rotor (1) in the tangential direction. 14. Apparatus according to any of the claims 8 to 13, character¬ ized in that at least one means (5) for increasing the gas pressure is ar¬ ranged on the rotor shaft to be driven by the rotor shaft (2). 15. Apparatus according to any of the claims 8 to 14, character¬ ized in that the means (10) for discharging the gas from the gap are formed as a radial channel system penetrating the stator (4). 16. Apparatus according to any of the claims 8 to 15, character- ized in that the revolving speed of the rotor (1) is so elected that the ratio A > 0.8, wherein the ratio A is the speed of sound in the pre-accele¬ rated gas / the peripheral speed of the rotor.

Inventors: HELEVIRTA KAUKO, LARJOLA JAAKKO, HELEVIRTA, KAUKO, LARJOLA, JAAKKO
Applicants: HELEVIRTA KAUKO, HIGH SPEED TECH LTD OY, LARJOLA JAAKKO, HELEVIRTA, KAUKO, HIGH SPEED TECH OY LTD., LARJOLA, JAAKKO
Application year: 1994
Main classification: H02K9
IPC codes: H02K1, H02K9